U.S. patent number 5,976,277 [Application Number 08/853,110] was granted by the patent office on 1999-11-02 for high speed tool steel, and manufacturing method therefor.
This patent grant is currently assigned to Pohang Iron & Steel Co., Ltd., Research Institute of Industrial Science & Technology. Invention is credited to Sang Ho Ahn, Eon Sik Lee, Woo Jin Park.
United States Patent |
5,976,277 |
Park , et al. |
November 2, 1999 |
**Please see images for:
( Certificate of Correction ) ** |
High speed tool steel, and manufacturing method therefor
Abstract
A high speed tool steel and a manufacturing method therefor are
disclosed, in which carbides are formed in the matrix in a uniform
manner, thereby obtaining a high toughness and a high abrasion
resistance. The high speed tool steel according to the present
invention includes a basic composition of W.sub.a Mo.sub.b Cr.sub.c
Co.sub.d V.sub.x C.sub.y Fe.sub.z where the subscripts meet in
weight %: 5.0%.ltoreq.a.ltoreq.7.0%, 4.0%.ltoreq.b.ltoreq.6.0%,
3.0%.ltoreq.c.ltoreq.5.0%, 6.5%.ltoreq.d.ltoreq.9.5%,
2.2%.ltoreq.x.ltoreq.8.3%, 1.1%.ltoreq.y.ltoreq.2.18%, and
66.52%.ltoreq.z.ltoreq.73.7%. The final structure has carbides
uniformly distributed within a martensite matrix, which are mainly
MC and M.sub.6 C carbides. The method includes the steps of melting
the above-defined alloy composition, gas spraying the melted alloy
to form a bulk material, heat treating the bulk material to
decompose the M.sub.2 C carbides to stabilize M.sub.6 C carbides
and hot working the heat treated bulk material to a desired
shape.
Inventors: |
Park; Woo Jin (Pohang,
KR), Lee; Eon Sik (Pohang, KR), Ahn; Sang
Ho (Pohang, KR) |
Assignee: |
Pohang Iron & Steel Co.,
Ltd. (KR)
Research Institute of Industrial Science & Technology
(KR)
|
Family
ID: |
26311509 |
Appl.
No.: |
08/853,110 |
Filed: |
May 8, 1997 |
Current U.S.
Class: |
148/543; 148/544;
148/545; 148/546; 148/547; 164/46; 164/461; 164/479; 164/97 |
Current CPC
Class: |
B22F
3/115 (20130101); C21D 8/005 (20130101); C22C
38/36 (20130101); C22C 38/22 (20130101); C22C
38/24 (20130101); C22C 38/30 (20130101); C22C
33/0285 (20130101); C21D 6/007 (20130101); C21D
2211/003 (20130101) |
Current International
Class: |
B22F
3/00 (20060101); B22F 3/115 (20060101); C22C
38/30 (20060101); C22C 33/02 (20060101); C22C
38/22 (20060101); C22C 38/24 (20060101); C21D
8/00 (20060101); C22C 38/36 (20060101); C21D
6/00 (20060101); B21J 001/00 (); C21D 005/00 () |
Field of
Search: |
;148/543,544,545,546,547
;164/46,461,97,479 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
48-020731 |
|
Mar 1973 |
|
JP |
|
55-38961 |
|
Mar 1980 |
|
JP |
|
63-199092 |
|
Aug 1988 |
|
JP |
|
63-235092 |
|
Sep 1988 |
|
JP |
|
647349 |
|
Feb 1979 |
|
SU |
|
Other References
Japanese Patent Laid-Open No. Sho 55-38961 Abstract, 1 p., (English
language), Mar. 18,1980. .
Korean Patent Laid-Open No. 96-21250 Abstract, 1 p.,(English
language), Jul. 18, 1996..
|
Primary Examiner: Ip; Sikyin
Attorney, Agent or Firm: Webb Ziesenheim Logsdon Orkin &
Hanson, P.C.
Claims
What is claimed is:
1. A method for manufacturing a high speed tool steel by applying a
spray casting method, comprising the steps of:
melting an alloy having a basic composition consisting essentially
of W.sub.a Mo.sub.b Cr.sub.c Co.sub.d V.sub.x C.sub.y Fe.sub.z
where the subscripts meet in weight %: 5.0%.ltoreq.a.ltoreq.7.0%,
4.0%.ltoreq.b.ltoreq.6.0%, 3.0%.ltoreq.c.ltoreq.5.0%,
6.5%.ltoreq.d.ltoreq.9.5%, 2.2%.ltoreq.x.ltoreq.8.3%,
1.1%.ltoreq.y.ltoreq.2.18%, and 66.52%.ltoreq.z.ltoreq.73.7%,
wherein x and y come within ranges of x.gtoreq.2.2, y.gtoreq.1.1,
y.gtoreq.-0.06+0.21x, y.ltoreq.2.8-0.13x, and y.ltoreq.1.26+0.2x so
as to form a molten alloy;
gas-spraying said molten alloy to form a bulk material having MC
and M.sub.2 C carbide structures therein and, wherein said molten
alloy is maintained at a temperature of 130.degree. C. to
290.degree. C. above a liquidus line temperature immediately before
said gas-spraying;
heat treating said bulk material to decompose the M.sub.2 C carbide
structures in the bulk material to obtain stabilized MC and M.sub.6
C carbide structures in the bulk material; and
hot working the heat treated bulk material to a desired shape.
2. The method as claimed in claim 1, wherein x and y come within
ranges of y.gtoreq.2.09-0.18x, y.gtoreq.0.06+0.21x,
y.ltoreq.2.8-0.13x and y.ltoreq.1.26+0.2x.
3. The method as claimed in claim 1, wherein said heat treatment
for decomposition of the M.sub.2 C carbide structures is carried
out at a temperature of 1000-1200.degree. C. for 1-16 hours.
4. The method as claimed in claim 1, wherein said hot working is
carried out at a temperature of 950-1150.degree. C.
5. The method as claimed in claim 4, wherein said hot working is
carried out with a forging ratio of 6 or more.
6. The method as claimed in claim 4, wherein said hot working is
carried out at a reduction ratio of 80% or more.
7. The method as claimed in claim 4, wherein said hot working is
carried out at an extrusion ratio of 10:1 or more.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for manufacturing a high
speed tool material for various tools. More specifically, the
present invention relates to a high speed tool steel and a
manufacturing method therefor, in which carbides are formed in the
matrix in a uniform manner, thereby obtaining a high toughness and
a high abrasion resistance.
2. Description of the Prior Art
Generally, a high speed tool steel is a high carbon alloy steel in
which carbide forming elements are contained in large amounts. For
example, one of them is W--Mo alloys, and others are W--Co alloys,
Mo--Co alloys, and W--Mo--Co alloys.
If the high speed tool steel is to withstand against a high speed
cutting operation, the abrasion resistance at a high temperature
has to be superior, and the toughness has to be sufficient. Such
mechanical properties of the high speed tool steel are decided by
the size, shape, distribution of the carbides within the alloy. The
carbides in high speed tool steels are classified by containing
metallic elements MC, M.sub.6 C, M.sub.2 C, M.sub.23 C.sub.6, and
M.sub.7 C.sub.3. MC is a carbide containing vanadium as the major
ingredient. M.sub.23 C.sub.6 is a carbide containing chrome as the
major ingredient, and M.sub.6 C and M.sub.2 C are carbides
containing tungsten and molybdenum as the major ingredients
respectively. Specifically, if the mechanical properties such as
abrasion resistance and toughness are to be superior, spherical
carbides having a size of 2-3 .mu.m should be uniformly
distributed.
Further, the high speed tool steels which have a manufacturing
history of more than 100 years show that their mechanical
properties are varied in accordance with the manufacturing
methods.
The method for manufacturing the high speed tool steels is
classified into an ordinary casting method and a powder
metallurgical method. In a high speed tool steel billet which is
manufactured by casting, coarse carbides are formed during the
casting, and these coarse carbides are non-uniformly distributed
within the billet, with the result that the workability becomes
bad, and that the toughness and the shock resistance become low.
Further, due to the growth of the coarse carbides and the severe
segregation of the micro-structure, the kinds and contents of the
alloy elements to be added are limited, this being a further
disadvantage.
On the other hand, in the case where the high speed tool steel is
manufactured by applying the powder metallurgical method, fine and
uniform carbides can be obtained owing to the rapid cooling.
Further, the amount of the alloy elements can be increased, and
therefore, a material having a high abrasion resistance can be
obtained.
For example, Japanese Patent Application Laid-open No. Sho-55-38961
discloses a method for manufacturing a high speed tool steel in
which the powder metallurgical method is applied while restricting
the content of tungsten. In this method, the growth of the M.sub.2
C carbide is inhibited, and instead, the growth of the MC and
M.sub.6 C carbides are promoted, with the result that toughness and
abrasion resistance are improved.
However, when manufacturing the high speed tool steel by applying
the powder metallurgical method, there is required a complicated
manufacturing process including the preparation of powder, a
particle size sorting, a canning, a degassing treatment, a
preform-making process, and a sintering process. Therefore, the
control of the manufacturing conditions is difficult, and
therefore, the manufacturing cost is increased.
Further, the M.sub.6 C carbides form carbide cells on the grain
boundaries within the powder, and the carbide cells grow during the
high temperature sintering so as to form continuous carbide cells.
If these are to be destroyed, a high forging ratio is required.
Further, in a coarse powder, there are generated the growth of
coarse M.sub.6 C carbides and a segregation phenomenon, and
therefore, toughness is adversely affected. Therefore, the control
of particle size becomes difficult.
Meanwhile, the present applicant filed a patent application (under
Korean Patent Application No. 94-38977) in which a method for
manufacturing a high speed tool steel by applying a spray forming
is disclosed unlike the casting and the powder metallurgical
methods.
In this spray forming method, the MC+M.sub.2 C carbides are formed,
and then, the M.sub.2 C carbides are made to be thermally
decomposed. Then a hot forging is carried out. This spray forming
method has many process advantages compared with the conventional
casting and powder metallurgical methods. However, this spray
forming method shows severe segregations, and therefore, it is
applied to a particular composition, while it has not been
commercialized.
SUMMARY OF THE INVENTION
The present invention is intended to overcome the above described
disadvantages of the conventional techniques.
Therefore it is an object of the present invention to provide an
Fe--C--V--W--Mo--Cr--Co high speed tool steel in which segregations
are inhibited unlike the conventional methods, so that it would be
suitable for the spray forming method, and that a high toughness
and a high abrasion resistance can be obtained.
It is another object of the present invention to provide a method
for manufacturing a high speed tool steel, in which a spray casting
method is applied, thereby obtaining a high toughness and a high
abrasion resistance.
It is still another object of the present invention to provided a
method for manufacturing a high speed tool steel, in which the
spray casting method is applied, and in which MC and M.sub.2 C
carbide structures are grown in the bulk material obtained from the
melt, and a thermal decomposition is carried out, thereby obtaining
a high speed tool steel containing finally stabilized MC and
M.sub.6 C carbides in a uniform manner.
It is still another object of the present invention to provided a
method for manufacturing a high speed tool steel, in which the
spray casting method is applied, and in which the formation of the
stabilized carbides can be easily controlled.
In achieving the above objects, the high speed tool steel according
to the present invention includes a basic composition of W.sub.a
Mo.sub.b Cr.sub.c Co.sub.d V.sub.x Fe.sub.z where the subscripts
meet in weight %: 5.0%.ltoreq.a.ltoreq.7.0%,
4.0%.ltoreq.b.ltoreq.6.0%, 3.0%.ltoreq.c.ltoreq.5.0%,
6.5%.ltoreq.d.ltoreq.9.5%, 2.2%.ltoreq.x.ltoreq.8.3%,
1.1%.ltoreq.y.ltoreq.2.18%, and 66.52%.ltoreq.z.ltoreq.73.7%; a
final structure having carbides uniformly distributed within a
martensite matrix; and main ingredients of said carbides being MC
and M.sub.6 C.
In another aspect of the present invention, the method for
manufacturing a high speed tool steel by applying a spray casting
method according to the present invention includes the steps of:
melting an alloy having a basic composition of W.sub.a Mo.sub.b
Cr.sub.c Co.sub.d V.sub.x C.sub.y Fe.sub.z where the subscripts
meet in weight %: 5.0%.ltoreq.a.ltoreq.7.0%,
4.0%.ltoreq.b.ltoreq.6.0%, 3.0%.ltoreq.c.ltoreq.5.0%,
6.5%.ltoreq.d.ltoreq.9.5%, 2.2%.ltoreq.x.ltoreq.8.3%,
1.1%.ltoreq.y.ltoreq.2.18%, and 66.52%.ltoreq.z.ltoreq.73.7%, so as
to form a melt; making a bulk material from said melt by applying a
gas-spraying process; carrying out a heat treatment for
decomposition on said bulk material; and carrying out a hot
working.
BRIEF DESCRIPTION OF THE DRAWINGS
The above object and other advantages of the present invention will
become more apparent by describing in detail the preferred
embodiment of the present invention with reference to the attached
drawings in which:
FIG. 1 is a graphical illustration showing carbon versus vanadium
in the alloy of the present invention and a comparative alloy;
FIG. 2 is a photograph showing the billet casting structure of the
alloy of the present invention and the comparative alloy;
FIG. 3 is a photograph showing the carbide structure of the alloy
of the present invention after carrying out a heat treatment for
decomposition;
FIG. 4 illustrates a case in which the alloy of FIG. 2 is
hot-worked; and
FIG. 5 is a photograph showing the casting structure of a billet
which has been spray-cast at a low temperature.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention provides an Fe--C--V--W--Mo--Cr--Co high
speed tool steel. The high speed tool steel according to the
present invention includes a basic composition of W.sub.a Mo.sub.b
Cr.sub.c Co.sub.d V.sub.x C.sub.y Fe.sub.z. Controls are made in
such a manner that the final structure should have carbides
uniformly distributed within a martensite matrix, and that
segregations should be prevented. For this purpose, the above
subscripts meet in weight %: 5.0%.ltoreq.a.ltoreq.7.0%,
4.0%.ltoreq.b.ltoreq.6.0%, 3.0%.ltoreq.c.ltoreq.5.0%.
6.5%.ltoreq.d.ltoreq.9.5%, 2.2%.ltoreq.x.ltoreq.8.3%,
1.1%.ltoreq.y.ltoreq.2.18%, and 66.52%.ltoreq.z.ltoreq.73.7%.
Specifically, tungsten and molybdenum are typical carbide forming
elements in the high speed tool steel. If their contents are
maintained at 6.0.+-.1.0 wt % and 5.0.+-.1.0 wt % respectively,
then acceptable levels of the mechanical properties can be
obtained.
Chrome and cobalt give a hardening effect to the high speed tool
steel, and improve its hardness. If their contents are maintained
at 4.0.+-.1.0 wt % and 8.0.+-.1.5 wt % respectively, then
acceptable levels of the mechanical properties can be obtained.
Vanadium is an alloy element which is observed in MC, M.sub.6 C and
M.sub.2 C carbides, but the amounts of vanadium included in the
respective carbides are different from each other. That is, the MC
carbide contains the largest amount of vanadium, the next is the
M.sub.2 C carbide, and the least amount of vanadium is contained in
the M.sub.6 C carbide.
Therefore, in accordance with the content of vanadium, the carbides
to be formed are decided. Specifically, if the addition amount of
vanadium is less than 2.2 wt %, the growth of M.sub.2 C carbide is
inhibited, and somewhat coarse MC and M.sub.6 C carbides are grown.
These carbides are uncontrollable during the heat treatment, and
after a hot working, the carbides are not fine and not uniformly
distributed, thereby lowering the toughness.
On the other hand, if vanadium is added more than 8.3 wt %, then
the MC carbide is formed in a large amount so as to deplete the
residual carbon amount within the melt. Consequently, the formation
of the M.sub.2 C carbide is inhibited, and the MC carbide is grown
into a coarse form, with the result that the final mechanical
properties are aggravated.
Therefore, the content of vanadium should be preferably maintained
at 2.2-8.3 wt %. For the formation of more uniform carbides, the
content of vanadium should be preferably maintained at more than
4.0 wt % and less than 8.0 wt %.
If the carbon content is insufficient, the cast structure after the
spray casting becomes not the MC+M.sub.2 C carbide structure but
the MC+M.sub.6 C carbide cells, with the result that the final
structure cannot be formed into the MC+M.sub.6 C carbide
structure.
On the other hand, if the carbon content is excessive, then a large
amount of coarse primary MC carbide is formed during the
solidification. Further, due to the formation of the MC carbide,
the amount of vanadium is exhausted, and consequently, the
formation of the M.sub.2 C carbide is inhibited. As a result, the
toughness and the abrasion resistance of the steel are lowered.
Therefore, basically the carbon content should be preferably
maintained at 1.1-2.1 wt %.
Meanwhile, the optimum carbon content should be decided in
accordance with the carbide forming elements contained in the high
speed tool steel, so that the high speed tool steel would be
suitable for the spray casting method. That is, the carbon content
is decided in accordance with which one of M.sub.2 C and M.sub.6 C
becomes the main ingredient. If the high speed tool steel is to be
made suitable for the spray casting, the M.sub.6 C carbide which
has a stable phase should be inhibited, and the M.sub.2 C carbide
which has a meta stable phase should be promoted. Therefore, The
carbon content is decided in such a manner that the M.sub.2 C
carbide of the meta stable phase as the eutectic carbide within the
spray cast structure should become the main ingredient.
For this purpose, in the high speed tool steel of the present
invention, the relationship between the content of carbon and that
of vanadium is important.
FIG. 1 is a graphical illustration showing carbon versus vanadium
in the alloy of the present invention and a comparative alloy.
That is, even if a sufficient amount of carbon is added, if
vanadium is insufficient (as shown by the X mark in FIG. 1), then
the primary MC carbide is excessively formed, with the result that
vanadium is exhausted in the melt, and that the formation of the
M.sub.2 C carbide is inhibited. Further, in the case where too much
vanadium is added, if carbon is insufficient (as shown by the (X)
mark in FIG. 1), then a delta (.delta.) ferrite is formed, with the
result that the hardening capability is lowered.
Considering such a relationship between carbon and vanadium, the
contents of carbon and vanadium should come within the following
ranges as shown in FIG. 1. That is, the ranges are x.gtoreq.2.2,
y.gtoreq.1.1, y.gtoreq.-0.06+0.21x, y.ltoreq.2.8-0.13x, and
y.ltoreq.1.26+0.2x (the A-B-E-D-C region of FIG. 1). More
preferably, the contents of carbon and vanadium should come within
the ranges of y.gtoreq.2.09-0.18x, y.gtoreq.0.06+0.21x,
y.ltoreq.2.8-0.13x and y.ltoreq.1.26+0.2x (the B-C-D-E region of
FIG. 1).
The final structure of the high speed tool steel according to the
present invention thus composed is converted into a martensite
matrix through a hardening treatment. Within the matrix, there are
formed MC and M.sub.6 C carbides. The size of the carbides is about
3 .mu.m, and the carbides are very much uniformly distributed.
Now the method for manufacturing the high speed tool steel
according to the present invention will be described.
The spray casting according to the present invention is carried out
in the following manner. That is, a melt within a tundish is
sprayed by means of a gas jet so as to make the sprayed melt
collided with a substrate. In this manner, a liquid state of about
50-70% is maintained, and a bulk material having the form of billet
or the like is produced.
When the spray casting process of the present invention is applied
to the high speed tool steel, first the composition of the melt is
adjusted to the ranges described above.
Thus, the melt which is composed as described above is sprayed into
a certain mold by the help of the gas jet, and the spray-cast bulk
material thus formed is made to have the MC and M.sub.2 C carbide
structures. Then a heat treatment for decomposition into MC and
M.sub.6 C is carried out, and then, a hot working is carried out to
make the fine final carbides distributed in a uniform manner.
The formations of the MC, M.sub.2 C and M.sub.6 C carbides which
are observed in the high speed tool steel of W.sub.a Mo.sub.b
Cr.sub.c Co.sub.d V.sub.x C.sub.y Fe.sub.z after its manufacture as
described above are closely related not only to the alloy
composition but also to the total amount of heat introduced into
the bulk material. The typical process condition which is related
to the introduced heat is the temperature of the melt.
When the melt temperature is too low, the carbides of the spray
cast microstructure are MC+M.sub.6 C and the yield efficiency of
the process also are reduced because a lot of impinging colder
droplets tend to bounce off without deposition. On the contrary,
when the melt temperature is too high, the ejection of liquid
surface on the top of bulk materials(growing billets) occurs by
external forces due to substrate rotation and impingement of the
gas jet, resulting in significantly low yield efficiency. In
addition, higher melt temperatures cause colony of coarser
MC+M.sub.2 C carbides which is similar to that obtained by the
conventional ingot casting and thus good mechanical properties
cannot be expected. Therefore, there should be a melt temperature
range at which MC+M.sub.2 C carbides are formed without sacrificing
the yield efficiency during spray casting.
Accordingly, if the carbides of the spray cast structure are to be
controlled into the MC+M.sub.2 C carbides, the temperature of the
melt has to be controlled. In the present invention, the
temperature of the melt immediately before the spraying should be
maintained higher than the liquidus line temperature preferably by
130-290.degree. C.
In the present invention, the liquidus line temperature of the high
speed tool steel of W.sub.a Mo.sub.b Cr.sub.c Co.sub.d V.sub.x
C.sub.y Fe.sub.z can be calculated based on the following
definition.
where all the contents of the elements are shown in weight %.
The temperature of the melt is controlled in this manner, and then,
the spraying is carried out. Under this condition, the spraying
conditions are the usually practiced ones. For example, the
desirable spraying conditions are as follows. That is, the melt
orifice diameter of the tundish is decided to be 3-9 mm, and the
melt droplet flight distance is decided to be 400-700 mm. The
primary gas pressure and the secondary gas pressure of the gas
nozzles are decided to be 1.5-4.5 bars and 5-10 bars respectively,
and the scanning frequency of the respective nozzles is decided to
be 12-18 cycles/sec.
In the spray cast alloy such as the billet which is obtained
through the above described spray conditions, there are contained
MC+M.sub.2 C carbides. The M.sub.2 C carbide begins to be
decomposed near 900.degree. C., but if it is to be decomposed
suitably for a hot fabricating process, either the maintaining time
has to be extended or the decomposing temperature has to be raised.
However, in the range of 900-1000.degree. C., several scores of
hours of maintaining time is required, and this gives an
inefficiency commercially. Meanwhile, at a temperature exceeding
1200.degree. C., the carbide is abnormally grown or redissolved,
thereby rather inviting the lowering of the toughness.
The temperature of the carbide decomposing heat treatment which is
suitable for the manufacture of the W.sub.a Mo.sub.b Cr.sub.c
Co.sub.d V.sub.x C.sub.y Fe.sub.z high speed tool steel should be
preferably limited to 1000-1200.degree. C. Specifically, if the
temperatures of the decomposing heat treatment are 1000.degree. C.,
1050.degree. C., 1100.degree. C., 1150.degree. C. and 1200.degree.
C., then the suitable maintaining time periods are 16, 8, 4, 2 and
1 hours.
The bulk material which has been thermally decomposed after being
spray-cast has to be made to undergo a hot working, so that the
structure of the carbides would become finer.
The hot working which is carried out in the present invention may
be any one of hot forging, hot rolling, and hot extrusion. The
important thing is to maintain the hot working temperature at
950-1150.degree. C. Due to a contact between the worked material
and the die during the hot working, the temperature of the surface
of the material greatly drops below the internal temperature, with
the result that there occurs a difference of the plasticity between
the surface and the internal region. If the hot working temperature
exceeds 1150.degree. C., the difference is significantly increased,
resulting in that severe cracks are formed on the surface and edges
of the material.
Meanwhile, if the temperature of the material is below 950.degree.
C., an insufficiency of the plasticity occurs due to the increase
in the deformation resistance of the material, and therefore, an
efficient hot working becomes impossible. Under this condition, if
an excessive load is imposed, then cracks are formed on the
material. Accordingly, the hot working temperature should be
maintained at the above mentioned level.
In the case where the hot forging is used, if the carbides are to
be uniformly distributed, the forging ratio needs to be 6 or
more.
In the case of the hot rolling, the reduction ratio should be 80%
or more, while in the case of the hot extrusion, the extrusion
ratio should be 10:1 or more. Then the resultant effect will be
almost the same.
After the hot working, an austenizing treatment is carried out, and
a hardening treatment in the form of a tempering is carried out.
Then the structure of the matrix can be converted into a
martensite.
If the above described melt temperature conditions for the W.sub.a
Mo.sub.b Cr.sub.c Co.sub.d V.sub.x C.sub.y Fe.sub.z high speed tool
steel are properly controlled during the spray casting, then the
carbide structures are made to contain only MC and M.sub.2 C. Then
if they are properly made to undergo a heat treatment for
decomposition, and if a hot working is carried out, then there can
be obtained a high toughness and high abrasion resistance W.sub.a
Mo.sub.b Cr.sub.c Co.sub.d V.sub.x C.sub.y Fe.sub.z high speed tool
steel in which fine MC carbides and fine M.sub.6 C carbides are
uniformly distributed.
Now the present invention will be described based on actual
examples.
EXAMPLE 1
Alloy systems were prepared which were composed of in weight 6.0%
of W, 5.0% of Mo, 4.0% of Cr and 8.0% of Co, carbon and vanadium
being contained as shown in Table 1 below. Then the alloys were
melted in an induction furnace under the external atmosphere, and
billets were manufactured by using a spray casting apparatus. As to
the melt temperature, first the liquidus line temperature was
calculated based on Formula 1, and then, the actual temperatures
were maintained at levels higher than the liquidus line by
130-290.degree. C. The respective alloys thus manufactured were
subjected to heat treatments at a temperature of 1200.degree. C.
for 1 hour. Then they were hot-forged with a forging ratio of 6 or
more. Then they were made to undergo hardening heat treatments, and
then, the mechanical properties were measured, the measured results
being shown in Table 1 below. The hardening heat treatments were
carried out in such a manner that first the alloys were all
austenized at a temperature of 1180.degree. C., then were
oil-quenched, and then, were tempered 3 times for one hour each
time at a temperature of 560.degree. C.
The alloys which have undergone the hardening heat treatments were
evaluated as to their hardness, their bend strength, and their
abrasion resistance. In measuring the hardness, first test
specimens of 20.times.20.times.20 (mm) were prepared, and then a
Rockwell hardness tester (C scale, diamond indenter, 150 Kgf) was
used. In measuring the bend strength, test specimens of
6.35.times.6.35.times.40.68 (mm) were prepared, and then, 3-point
bend tests were carried out at a speed of 0.5 mm/min. In evaluating
the abrasion resistance, test pieces of 30.times.30.times.5 (mm)
were prepared, and then, tests were carried out with a load of 100
Kgf by using the SKD61 alloy as the counter part material.
In Table 1 below, the conventional materials are the M2 and ASP30
casting alloys and the ASP30 powder metallurgical alloy. That is,
the conventional examples 1 and 2 were the M2
(6%W-5%Mo-4%Cr-2%V-0.85%C) and ASP
(6.4%W-5%Mo-4.2%Cr-8.5%Co-3.1%V-1.3%C) high speed tool steels which
were manufactured by the ordinary casting method, and they had a
composition similar to that of the present invention. The
conventional example 3 was a high speed tool steel which was
manufactured by the powder metallurgical method, and which was an
ASP alloy having the above indicated composition. They were all
made to undergo the hot workings and the hardening treatments in
the same manner as that of the present invention, and the
mechanical properties were measured in the same way.
TABLE 1
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abrasion alloy compo- melt resistanceg test sition (wt %) liquidus
hardness/ature strength (load decrease)/ piece No. V C Fe line
(.degree. C.) (.degree. C.) (HRc) (GPa) (mg/km)
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comparative 1.7 1.22 bal. 1392.8 1602.8 .+-. 80 63.2 2.41 87.7
example 1 comparative 2.1 1.45 bal. 1366.5 64.16.5 .+-. 80 86.8
example 2 comparative 2.9 0.92 bal. 1421.9 63.61.9 .+-. 80 79.4
example 3 comparative 2.9 1.05 bal. 1408.5 62.88.5 .+-. 80 90.0
example 4 comparative 3.3 1.95 bal. 1305.2 64.25.2 .+-. 80 84.3
example 5 comparative 5.1 1.02 bal. 1407.2 64.27.2 .+-. 80 87.8
example 6 comparative 5.0 2.15 bal. 1274.8 66.84.8 .+-. 80 52.4
example 7 comparative 5.9 1.01 bal. 1406.7 64.36.7 .+-. 80 56.3
example 8 comparative 6.1 1.15 bal. 1391.5 64.51.5 .+-. 80 50.5
example 9 comparative 7.1 1.29 bal. 1375.4 64.75.4 .+-. 80 53.2
example 10 inventive 2.5 1.32 bal. 1380.2 66.30.2 .+-. 80 83.5
example 1 inventive 2.9 1.14 bal. 1399.0 65.89.0 .+-. 80 58.2
example 2 inventive 3.1 1.26 bal. 1385.5 66.55.5 .+-. 80 56.5
example 3 inventive 3.0 1.31 bal. 1380.3 66.90.3 .+-. 80 54.9
example 4 inventive 3.2 1.45 bal. 1364.3 67.74.3 .+-. 80 53.4
example 5 inventive 3.1 1.57 bal. 1350.8 68.40.8 .+-. 80 52.1
example 6 inventive 3.1 1.69 bal. 1336.8 65.66.8 .+-. 80 73.4
example 7 inventive 3.1 1.77 bal. 1327.4 66.07.4 .+-. 80 69.5
example 8 inventive 4.0 1.24 bal. 1386.0 67.56.0 .+-. 80 53.0
example 9 inventive 4.1 1.45 bal. 1362.5 69.02.5 .+-. 80 52.1
example 10 inventive 5.0 1.12 bal. 1396.9 67.16.9 .+-. 80 44.5
example 11 inventive 4.9 1.28 bal. 1379.8 68.49.8 .+-. 80 41.8
example 12 inventive 5.1 1.53 bal. 1351.4 68.81.4 .+-. 80 40.4
example 13 inventive 5.1 1.84 bal. 1315.0 69.55.0 .+-. 80 39.0
example 14 inventive 6.0 1.92 bal. 1305.4 70.75.4 .+-. 80 37.5
example 15 inventive 6.0 1.29 bal. 1376.5 68.06.5 .+-. 80 45.5
example 16 inventive 5.9 1.67 bal. 1333.6 71.63.6 .+-. 80 39.4
example 17 inventive 6.1 1.88 bal. 1308.1 72.88.1 .+-. 80 36.5
example 18 inventive 7.1 1.43 bal. 1358.8 69.38.8 .+-. 80 38.4
example 19 inventive
7.2 1.51 bal. 1349.5 71.39.5 .+-. 80 40.6 example 20 inventive 7.2
1.73 bal. 1323.9 72.53.9 .+-. 80 39.5 example 21 conventional 2.0
0.85 bal. M2 alloy made by 65.8 3.12 109.0 example 1 ordinary
casting method conventional 3.1 1.3 bal. ASP30 alloy made by 83.5
example 2 ordinary casting method conventional 3.1 1.3 bal. ASP30
alloy made by 55 example 3 ordinary powder metallurgical method
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As shown in Table 1 above, in the cases of the inventive examples
1-21 which were manufactured according to the present invention,
the overall mechanical properties were superior compared with the
comparative examples 1-10. Particularly, when the inventive
examples were compared with the cast materials of the conventional
examples 1 and 2, the hardness was similar to each other, but the
bending strength and the abrasion resistance of the inventive
examples were more than twice those of the conventional examples 1
and 2. Meanwhile, when the inventive examples were compared with
the conventional example 3 which was manufactured by the usual
powder metallurgical method, the hardness and the bending strength
were similar to each other, but the abrasion resistance of the
present invention was superior over that of the conventional
example 3.
Meanwhile, the casting structures of the billets were observed, and
the typical structure is illustrated in FIG. 2. The spray-cast
structures of the billets showed two types of carbide structures.
The first was the MC+M.sub.2 C carbide structures as shown in FIG.
2A. That is, they were carbide cells composed of the spherical MC
carbides and the rod shaped M.sub.2 C carbides (inventive
examples). The second was the MC+M.sub.6 C carbide structures as
shown in FIG. 2B. That is, they were carbide cells composed of the
spherical MC carbides and the spherical M.sub.6 C carbides
(comparative examples).
FIG. 1 corresponds to the alloys of Table 1. That is, it
illustrates the alloys of the inventive examples having the casting
structure of MC+M.sub.2 C and the alloys of the comparative
examples having the structure of MC+M.sub.6 C in accordance with
the contents of carbon and vanadium.
As shown in FIG. 1, the internal region of the polygon illustrates
the region of the present invention in which the spray-cast
structure is the MC+M.sub.2 C carbides. The outer region
illustrates the region of the comparative examples in which the
spray-cast structure is the MC+M.sub.6 C carbides. Here, it can be
known that the high speed tool steel of the present invention first
has to have a casting structure of the MC+M.sub.2 C carbides. After
all, here it has been confirmed that the relationship between the
carbon ingredient y and the vanadium ingredient x should be as
follows. That is, x.gtoreq.2.2, y.gtoreq.1.1, y.gtoreq.0.06+0.21x,
y.ltoreq.2.8-0.13x and y.ltoreq.1.26+0.2x have to be satisfied.
Further during the heat treatment for decomposition of the
carbides, if fine carbides are to be grown, then x and y should
preferably satisfy the relationships of y.gtoreq.2.09-0.18x,
y.gtoreq.-0.06+0.21x, y.ltoreq.2.8-0.13x, y.ltoreq.1.26+0.2x.
FIG. 3 illustrates the micro-structure of the M.sub.2 C carbides of
the inventive example 11 after the heat treatment for
decomposition. By the heat treatment for decomposition, the rod
shaped M.sub.2 C carbides were decomposed into the MC carbides and
the M.sub.6 C carbides of less than 2 .mu.m. On the other hand, in
the case of the comparative examples, they had the MC+M.sub.6 C
carbide structures, and the carbide structures were not decomposed
even with the heat treatment of decomposition.
FIG. 4 illustrates the case in which the billets of FIG. 2 were
hot-worked after carrying out a decomposing heat treatment. FIG. 4A
illustrates a micro-structure after hot-working the material of
FIG. 2A, while FIG. 4B illustrates a micro-structure after
hot-working the material of FIG. 2B.
As shown in FIG. 4A, the alloys of the present invention showed
micro-structures in which the spherical fine MC and M.sub.6 C
carbides were uniformly distributed. On the contrary, as shown in
FIG. 4B, the comparative alloys were irregular in the size and
distribution of the carbides.
EXAMPLE 2
An alloy which has the composition of the inventive example 10 was
formed into billets in the same manner as that of Example 1, except
that the melt temperature was 1460.degree. C., this being lower
than the melt temperature of Table 1. The structure of the billet
thus manufactured is shown in FIG. 5 (comparative example 11).
As shown in FIG. 5, in the case of the comparative example 11 which
was manufactured at a temperature lower than that of the present
invention, the carbide structure was composed of carbide cells of
the MC carbides and the M.sub.6 C carbides. Such a carbide
structure includes coarse M.sub.6 C carbides non-uniformly
distributed even after the hot working.
Actually, when this alloy was hot-worked, the properties were a
hardness of 63.4 HRc, a bending strength of 2.24 GPa and an
abrasion resistance of 90.1 mg/Km. That is, when the melt
temperature was lower than that of the present invention, the
uniform structure which was seen in the present invention could not
be obtained with any conditions of heat treatment and hot working,
but the toughness and the abrasion resistance were markedly
aggravated. Consequently, it was confirmed that the melt
temperature was important in manufacturing the high speed tool
steel of the present invention.
EXAMPLE 3
Billets were manufactured in the same way as that of Example 1,
except that the alloy composition was in weight %: 6.5% of W, 5.0%
of Mo, 4% of Cr, 3.1% of V, 8% of Co, and a balance of Fe, in
addition to 1.42% of C (inventive example 22), and 1.05% of C
(comparative example 12). The billets were manufactured by using a
spray-casting apparatus, and then, their mechanical properties were
measured.
In the case of the inventive example 22, the evaluation of the
mechanical properties showed a hardness of 67.2 HRc, a bending
strength of 4.47 GPa, and an abrasion resistance of 55.0 mg/Km. In
the case of the comparative example 12, the evaluation showed a
hardness of 63.1 HRc, a bending strength of 2.69 GPa and an
abrasion resistance of 94.2 mg/Km. That is, the inventive example
22 showed to be superior over the comparative example 12 in all the
mechanical properties.
According to the present invention as described above, there is
provided a high toughness and high abrasion resistance W.sub.a
Mo.sub.b Cr.sub.c Co.sub.d V.sub.x C.sub.y Fe.sub.z high speed tool
steel in which fine carbides are uniformly distributed.
Particularly, this high speed tool steel can be manufactured by the
spray casting method at a cheap cost compared with the conventional
casting and/or powder metallurgical method. Thus, the conventional
high prestige high speed tool steel component which could be
obtained by using only the expensive powder metallurgical material
can be substituted by a cheap material having the same mechanical
properties. At the same time, the application of the high
performance high speed tool steel can be widened.
* * * * *